Zeolite refers to a generic term for a group of crystalline aluminosilicate. Because sites around aluminum in the framework of aluminosilicate bear negative charges, cations for charge balance are present in the pores, and the remaining space of the pores is usually filled with water molecules. The three-dimensional pore structure, shape and size of zeolite vary depending upon the type of zeolite, but the pore diameter usually corresponds to the molecular scale. Therefore, zeolite is also called “molecular sieve”, because it has size selectivity or shape selectivity for molecules entering the pores depending on the type of zeolite.
On the other hand, many zeotype molecular sieves are known in which silicon (Si) and aluminum (Al) atoms constituting the framework structure of zeolite are partially or wholly replaced by various other elements. Examples of known zeotype molecular sieves include porous silicalite-based molecular sieves free of aluminum, AlPO4-based molecular sieves in which silicon is replaced by phosphorus (P), and other zeotype molecular sieves obtained by substituting a portion of the framework of such zeolite and zeotype molecular sieves with various metal atoms such as Ti, Mn, Co, Fe and Zn. These zeotype molecular sieves are materials derived from zeolites, and do not belong to the zeolite group based on the mineralogical classification but are commonly called as zeolites in the art.
Accordingly, the term “zeolite” as used herein is intended to include the above-mentioned zeotype molecular sieves in a broad sense.
Zeolites with an MFI structure are most actively used and include the following types:
1) ZSM-5: MFI zeolite in which silicon and aluminum are present in a specific ratio;
2) Silicalite-1: zeolite composed only of silica; and
3) TS-1: MFI zeolite in which aluminum atoms are partially replaced by titanium (Ti) atoms.
The structure of an MFI zeolite is depicted in FIG. 1. In the MFI zeolite, elliptical pores (0.51 nm×0.55 nm) are connected in a zigzag configuration to form channels extending in the a-axis direction, and substantially circular pores (0.54 nm×0.56 nm) linearly extend in the b-axis direction to form straight channels. No channels remain open in the c-axis direction.
Powdered MFI zeolites are very widely used in household and industrial applications, including petroleum cracking catalysts, adsorbents, dehydrating agents, ion exchangers, gas purifiers, etc. MFI zeolite thin films formed on porous substrates, such as porous alumina, are widely used as membranes through which molecules can be separated on the basis of size. Furthermore, MFI zeolite thin films can find application in a wide range of fields, for example, second- and third-order nonlinear optical thin films, three-dimensional memory materials, solar energy storage devices, electrode auxiliary materials, carriers of semiconductor quantum dots and quantum wires, molecular circuits, photosensitive devices, luminescent materials, low dielectric constant (k) thin films, anti-rusting coatings, etc.
As described above, the pore shape, size and channel structure of MFI zeolites vary depending on the crystal direction.
Meanwhile, methods for producing MFI zeolite thin films on substrates such as glass plates are broadly divided into a primary growth method and a secondary growth method. According to the primary growth method, a substrate is dipped in a gel for the synthesis of MFI zeolite without any pretreatment, and then spontaneous growth of an MFI zeolite film on the substrate is induced. Generally, the gel for synthesis contains tetrapropylammonium hydroxide (TPAOH). In this case, b-axis-oriented MFI zeolite crystals grow perpendicular to the substrate at the initial stage of the reaction. At this time, a-axis oriented crystals begin to grow parasitically from central portions of most of the crystals grown on the glass plate. With the passage of time, the crystals grow in various directions, and as a result, the final thin film has various orientations. The randomly oriented MFI zeolite thin film is useful in some applications, but its applicability is limited. Particularly, when the randomly oriented MFI zeolite thin film is applied as a membrane for the separation of molecules, the molecular permeability, which is one of the most important factors in the molecular separation, is markedly reduced. When organic bases other than TPAOH are used in the primary growth method, no MFI zeolite thin film grows on the substrate. The secondary growth method is an alternative way to overcome the drawbacks of the primary growth method.
According to the secondary growth method, a substrate, to which MFI zeolite crystals are previously attached, is dipped in an MFI zeolite synthesis gel, and then the reaction is allowed to proceed to form an MFI zeolite thin film. The MFI zeolite crystals attached to the substrate act as seeds. The orientation of the MFI zeolite crystals, previously attached on the substrate, plays a very important role in determining the orientation of the MFI zeolite thin film to be produced later. For example, when the a-axes of the MFI zeolite seed crystals are oriented perpendicular to the substrate, the a-axes of the MFI zeolite thin film, formed from the oriented seed crystals, tend to be oriented perpendicular to the substrate, and when the b-axes of the MFI zeolite seed crystals are oriented perpendicular to the substrate, the b-axes of the MFI zeolite thin film, formed from the oriented seed crystals, tend to be oriented perpendicular to the substrate.
However, the orientation of the resulting zeolite thin film is highly sensitive to an organic base contained in the synthesis gel rather than to the orientation of the seed crystals. For example, the MFI synthesis gel which has been used in the secondary growth method usually contains TPAOH. In this case, even when the MFI zeolite seed crystals are attached to the substrate such that the a- or b-axes are oriented perpendicular to the substrate, the orientation of the resulting MFI zeolite thin film varies randomly.
Throughout the specification, a number of publications and patent documents are referred to and cited. The disclosure of the cited publications and patent documents is incorporated herein by reference in its entirety to more clearly describe the state of the related art and the present disclosure.